Polycarbonate polymers are known as being excellent molding materials since products made therefrom exhibit such properties as high impact strength, toughness, high transparency, wide temperature limits (high impact resistance below -60.degree. C. and a UL thermal endurance rating of 115.degree. C. with impact), good dimensional stability, good creep resistance, and the like. It would be desirable to add to this list of properties that of improved flame retardance so that products made from such polycarbonate polymers could be safely used by the consumer and also meet the increasing requirements of certain flame retardant criteria being established by local and federal government agencies as well as the manufacturers of such products. It would also be desirable to improve the moisture barrier property of such polycarbonates thereby enabling them to be used in a wider range of product applications.
It is known to obtain polycarbonates which contain halogenated monomers as their main, polymeric building blocks. For example, U.S. Pat. No. 3,028,365 discloses a host of polycarbonate compositions including tetrabromobisphenol-A and a dichloromethylenediphenol monomer, as well as processes for obtaining these polycarbonates.
U.S. Pat. No. 3,062,781 discloses that halogenated polycarbonates can be obtained by first halogenating a diphenol containing at least two halogen substituents. However, the only dihalogenated diphenol disclosed is dichlorobisphenol-A.
German Pat. No. P25 20 317.2 discloses that halogenated polycarbonates can be obtained by halogenating bisphenol-A (4,4'-isopropylidenediphenol) to produce a mixture of unreacted bisphenol-A and statistical mixtures of halogenated bisphenol-A (BPA). The halogenated bisphenols disclosed comprise, primarily, tri- and tetrahalogenated BPA.
In general, these prior art references recognize that flame retardance can be imparted to polycarbonates by halogenating the monomeric building blocks from which they are obtained. In addition, these references suggest that the greater the degree of halogenation of the monomer, the better will be the fire retardance imparted to the polymer. U.S. Pat. No. 3,062,781 also indicates that halogenated diphenols have reduced permeability to steam. However, none of these references discloses or suggests that a high molecular weight aromatic polycarbonate resin having improved flame retardance as well as improved water vapor transmission can be obtained from particular halogenated diphenols.